Man, that's a major flub on FedEx's part. Did they say what that means in terms of locating the lost package?
I think I mentioned this in another post, but I learned my lesson about trade-ins -- when possible, bring the trade-in to a physical carrier store and have them send it in. When I recently traded in my daughter's iPhone 8 for a 12, I mailed it in (since T-Mobile didn't have a 12 in stock at the time the transaction was made). It was working perfectly when I packed it, but they later claimed that the screen wasn't working. Fortunately, T-Mobile agreed to honor the trade-in after we asked the trade-in facility to produce photo evidence that the screen was faulty, and they couldn't. But the point is that T-Mobile told me that if their store had been the one to send it to the trade-in facility (after confirming it was in working order), then any problems with the trade-in would've been on T-Mobile, not me.